1. Field of the Invention
The present invention relates generally to earth-boring bits, especially the seal and lubrication systems for earth-boring bits of the rolling cutter variety. More particularly, the present invention relates to an improved O-ring and seal assembly for improving the abrasion resistance and wear characteristics of rolling cutter bits by more effectively retaining the lubricant around the journal bearings of such earth-boring bits.
2. Background Information
The rotary rock bit was an important invention that enabled the discovery of deep oil and gas reservoirs. Only soft earthen formations could be penetrated commercially with the earlier drag bit which drilled only a scant fraction of the depth and speed of the modern rotary rock bit. Modern rock bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of earth-boring bits of the rolling cutter variety.
The modern earth boring bit employs one or more, typically three, rolling cone cutters rotatably mounted thereon. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material. The rolling cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring.
As the cutters roll and slide along the bottom of the borehole, the cutters, and the shafts on which they are rotatably mounted, are subjected to large static loads from the weight on the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, most earth-boring bits are provided with precision-formed journal bearings and bearing surfaces, as well as sealed lubrication systems to increase drilling life of bits. The lubrication systems typically are sealed to avoid lubricant loss and to prevent contamination of the bearings by foreign matter such as abrasive particles encountered in the borehole. A pressure compensator system minimizes pressure differential across the seal so that lubricant pressure is equal to or slightly greater than the hydrostatic pressure in the annular space between the bit and the sidewall of the borehole.
Early earth boring bits had no seals or rudimentary seals with relatively short life, and, if lubricated at all, necessitated large quantities of lubricant and large lubricant reservoirs. Typically, upon exhaustion of the lubricant, journal bearing and bit failure soon followed. An advance in seal technology occurred with the "Belleville" seal, as disclosed in U.S. Pat. No. 3,075,781, to Atkinson et al. The Belleville seal minimized lubricant leakage and permitted smaller lubricant reservoirs to obtain acceptable bit life.
The seal disclosed by Atkinson would not seal lubricant inside a journal-bearing bit for greater than about 50-60 hours of drilling, on average. This was partially due to rapid movement of the cutter on its bearing shaft (cutter wobble), necessitated by bearing and assembly tolerances, which causes dynamic pressure surges in the lubricant, forcing lubricant past the seal, resulting in premature lubricant loss and bit failure.
Improvements in journal bearing seals, in later years, included bits employing anti-friction ball or roller bearing elements. However, it was the O-ring, journal bearing combination disclosed in U.S. Pat. No. 3,397,928, to Galle that unlocked the potential of the journal-bearing bit. Galle's O-ring-sealed, journal-bearing bit could drill one hundred hours or more in the hard, slow drilling of West Texas. The success of Galle's design was in part attributable to the ability of the O-ring design to help minimize the aforementioned dynamic pressure surges.
It was discovered relatively early on that ordinary O-ring seals capable or providing adequate sealing in less demanding environments are inadequate in rock bits. Refinements in O-ring technology included choice of materials, configuration of the annular channel or groove in which the O-ring is confined and the degree of squeeze or compression on the O-ring in the assembled bit.
Typical O-ring materials used in the prior art include butadiene acrylonitrile (Buna N) rubber and ethylene/propylene/diene/monomer (EPDM) rubber or polymethylene.
Butadiene acrylonitrile rubber is based on a copolymer of butadiene with varying amounts of acrylonitrile. EPDM rubbers are a basic class of synthetic rubbers which have a basic polymer backbone built of copolymerized ethylene and propylene molecules, and side chains containing a double bond usable for cross-linking in a vulcanization or final curing step. The side chain is derived from a non-conjugated diene, such as 1,4-hexadiene, which is copolymerized in proper amounts with ethylene and propylene.
Despite improvements in the materials available for use as the resilient seal member of rock bit bearing seals, a need exists for such materials which provide more effective seals for journal bearing rock bits under the rigorous conditions encountered in down-hole drilling.
A need also exists for seal assemblies for rotary rock bits which more effectively seal between a stationary and a rotating surface.
A need exists for an improved O-ring seal assembly for retaining the lubricant within the rock bit over a prolonged useful life and over a full range of operating speeds.
A need also exists for such a bearing seal assembly employing an O-ring which is resistant to hydrocarbons or other chemical compositions found in the downhole environment, which has a high heat resistance and which does not unduly deform under changing loads to permit lubricant to escape the bit or to allow the ingress of external fluids into the bit interior.